Apparatus for polishing thin, flat semiconductor wafers are well-known in the art. Such apparatus normally includes a polishing head which carries a membrane for engaging and forcing a semiconductor wafer against a wetted polishing surface, such as a polishing pad. Either the pad or the polishing head is rotated and oscillates the wafer over the polishing surface. The polishing head is forced downwardly onto the polishing surface by a pressurized air system or similar arrangement. The downward force pressing the polishing head against the polishing surface can be adjusted as desired. The polishing head is typically mounted on an elongated pivoting carrier arm, which can move the pressure head between several operative positions. In one operative position, the carrier arm positions a wafer mounted on the pressure head in contact with the polishing pad. In order to remove the wafer from contact with the polishing surface, the carrier arm is first pivoted upwardly to lift the pressure head and wafer from the polishing surface. The carrier arm is then pivoted laterally to move the pressure head and wafer carried by the pressure head to an auxiliary wafer processing station. The auxiliary processing station may include, for example, a station for cleaning the wafer and/or polishing head, a wafer unload station, or a wafer load station.
More recently, chemical-mechanical polishing (CMP) apparatus has been employed in combination with a pneumatically actuated polishing head. CMP apparatus is used primarily for polishing the front face or device side of a semiconductor wafer during the fabrication of semiconductor devices on the wafer. A wafer is “planarized” or smoothed one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer is polished by being placed on a carrier and pressed face down onto a polishing pad covered with a slurry of colloidal silica or alumina in deionized water.
CMP polishing results from a combination of chemical and mechanical effects. A possible mechanism for the CMP process involves the formation of a chemically altered layer at the surface of the material being polished. The layer is mechanically removed from the underlying bulk material. An altered layer is then regrown on the surface while the process is repeated again. For instance, in metal polishing, a metal oxide may be formed and removed separately.
A polishing pad is typically constructed in two layers overlying a platen with the resilient layer as the outer layer of the pad. The layers are typically made of polyurethane and may include a filler for controlling the dimensional stability of the layers. The polishing pad is usually several times the diameter of a wafer and the wafer is kept off-center on the pad to prevent polishing a non-planar surface onto the wafer. The wafer is also rotated to prevent polishing a taper into the wafer. Although the axis of rotation of the wafer and the axis of rotation of the pad are not collinear, the axes must be parallel.
In a CMP head, large variations in the removal rate, or polishing rate, across the whole wafer area are frequently observed. A thickness variation across the wafer is therefore produced as a major cause for wafer non-uniformity. In the improved CMP head design, even though a pneumatic system for forcing the wafer surface onto a polishing pad is used, the system cannot selectively apply different pressures at different locations on the surface of the wafer. The thickness difference between the highest point and the lowest point on the wafer is almost 2,000 angstroms, resulting in a standard deviation of 472 angstroms, or 6.26%. The removal rates obtained at the edge portions of the wafer are substantially higher than the removal rates at or near the center of the wafer. The thickness uniformity on the resulting wafer after the CMP process is poor.
Referring to FIG. 1A, a conventional CMP apparatus 50 includes a conditioning head 52, a polishing pad 56, and a slurry delivery arm 54 positioned over the polishing pad 56. The conditioning head 52 includes a conditioning disk 68 which is mounted on a conditioning arm 58 which is extended over the top of the polishing pad 56 for making a sweeping motion across the entire surface of the polishing pad 56. The slurry delivery arm 54 is equipped with slurry dispensing nozzles 62 which are used for dispensing a slurry solution on the top surface 60 of the polishing pad 56. Surface grooves 64 are further provided in the top surface 60 to facilitate even distribution of the slurry solution and to help entrapping undesirable particles that are generated by coagulated slurry solution or any other foreign particles which have fallen on top of the polishing pad 56 during a polishing process. The surface grooves 64, while serving an important function of distributing the slurry, also presents a processing problem when the pad surface 60 gradually wears out after prolonged use.
The conventional conditioning disk 68 may be of several different types. A conventional brazed grid-type conditioning disk is formed by embedding or encapsulating diamond particles in random spacings with each other in the surface of a stainless steel substrate. A conventional dia grid-type conditioning disk is formed by embedding cut diamonds at regular spacings in a nickel film coated onto the surface of a stainless steel substrate. The diamonds are typically coated with a diamond-like carbon (DLC) layer.
The CMP apparatus 50 typically further includes a polishing head 70 which is mounted on a rotatable shaft 72 above the top surface 60 of the polishing pad 56. As shown in FIG. 1B, the polishing head 70 holds and rotates a wafer 74 against the top surface 60 of the polishing pad 56 to polish the wafer 74. Before production wafers are polished using the CMP apparatus 50, time is typically allotted to warm the polishing pad 56 and facilitate flow of polishing slurry from a slurry container (not shown) to the slurry delivery arm 54. This enhances polishing uniformity among successive wafers polished on the apparatus 50.
The polishing pad 56 is a consumable item used in a semiconductor wafer fabrication process. Under normal wafer fabrication conditions, the polishing pad 56 is replaced after about 12 hours of usage. Polishing pads may be hard, incompressible pads or soft pads. For oxide polishing, hard and stiffer pads are generally used to achieve planarity. Softer pads are generally used in other polishing processes to achieve improved uniformity and smooth surfaces. The hard pads and the soft pads may also be combined in an arrangement of stacked pads for customized applications.
A problem frequently encountered in the use of polishing pads in oxide planarization is the rapid deterioration in oxide polishing rates with successive wafers. The cause for the deterioration is known as “pad glazing”, wherein the surface of a polishing pad becomes smooth such that slurry is no longer held in between the fibers of the pad. This physical phenomenon on the pad surface is not caused by any chemical reations between the pad and the slurry.
To remedy the pad glazing effect, numerous techniques of pad conditioning or scrubbing have been proposed to regenerate and restore the pad surface and thereby restore the polishing rates of the pad. The pad conditioning techniques include the use of silicon carbide particles, diamond emery paper, blade or knife for scraping or scoring the polishing pad surface. The goal of the conditioning process is to remove polishing debris from the pad surface and re-open pores in the pad by forming micro-scratches in the surface of the pad for improved pad lifetime. The pad conditioning process can be carried out either during a polishing process, i.e. known as concurrent conditioning, or after a polishing process. While the pad conditioning process improves the consistency and lifetime of a polishing pad, a conventional conditioning disk is frequently not effective in conditioning a pad surface after repeated usage.
Prior to the CMP operation, each wafer is typically subjected to a CVD (chemical vapor deposition) or other process to sequentially deposit material layers thereon. These layers include conductive layers, insulative layers, via layers and IMD (intermetal dielectric) layers, for example. The subsequent CMP operation polishes each layer to the desired thickness for precise dimensional control of the device components to be fabricated in the layers. However, in a modern semiconductor fabrication facility, wafers in different lots are frequently processed in different CVD chambers, which vary among each other in the thickness of a given layer that is deposited on a wafer.
The CMP apparatus carries out polishing operations on each wafer according to a recipe which is programmed into the controller (not shown) for the CMP apparatus. Because the layers on each wafer must typically be polished to different thicknesses, each layer on the wafer has its own polishing recipe. The polishing recipe includes such variables as down pressure and polish time. However, due to variations in layer thicknesses between wafer lots processed in different CVD chambers, the CMP apparatus, operating according to a given polishing recipe for each layer, has a tendency to overpolish some layers and underpolish other layers in a wafer, resulting in layers of imprecise thickness on the wafer. For this reason, for a given layer on each wafer of a given lot of wafers, each polishing recipe is programmed with a compensated removal rate to compensate for this polishing imprecision and facilitate polishing of each layer to a thickness which is as precise as possible.
The conventional polishing compensation process described above usually involves the use of a computer server and supporting software that includes first and second tables to aid personnel in the selection of the correct polishing recipe having the appropriate compensated removal rate for each of the layers on each wafer in a lot. The first table includes a sequential listing of the various lots of wafers, each of which is paired with the various layers to be polished on each wafer. An example of such a table is shown below as Table I.
TABLE IProduct ID (lot)LayerTMA001VIA1 CMPTMA001VIA2 CMPTMA001VIA3 CMPTMA001VIA4 CMPTMA002VIA1 CMPTMA002VIA2 CMPTMA002VIA3 CMPTMA002VIA4 CMP
The second table displayed on the server includes a sequential listing of high and low limits for the thickness of each layer in Table I to be polished, paired with the appropriate polishing recipe to obtain a target layer thickness that lies within the desired range. An example of such a table is shown below as Table II.
TABLE IILower THK limitHigher THK limitRecipe22,00022,200IMD84.CAS22,20022,400IMD86.CAS22,40022,600IMD88.CAS22,60022,800IMD9.CAS22,80023,000IMD92.CAS23,00023,200IMD94.CAS23,20023,400IMD96.CAS23,40023,600IMD98.CAS
One of the problems which is inherent in the conventional, table-based method of compensating for CMP polishing imprecision is that, in attempts to achieve a layer thickness which is as close as possible to the target layer thickness, the method is capable of corrective over-polishing or under-polishing only by increments. As can be seen from Table II, each layer on a wafer can be over-polished or under-polished typically by 200 angstroms to achieve a layer thickness which is as close as possible to the target layer thickness for the layer. As an example, a normal polishing recipe for a given layer on a wafer may result in a layer which is 110 angstroms thicker than the target layer thickness. Using the compensation removal rate, the recipe, therefore, operates the CMP apparatus to overpolish the layer and remove an additional 200 angstroms from the wafer. The result is a layer which is 90 angstroms (200–110) thinner than the target layer thickness.
As another example, a normal polishing recipe for a given layer on a wafer may result in a layer which is 280 angstroms thinner than the target layer thickness. Using the compensation removal rate, the compensation recipe operates the CMP apparatus to underpolish the layer to leave an additional 200 angstroms on the wafer. The result is a layer which is 80 angstroms (280–200) thicker than the target layer thickness.
Another problem inherent in the conventional method is that the software required for the program occupies an inordinately large space on the server's hard drive. Accordingly, a new and improved method is needed to compensate for imprecisions in the CMP polishing of wafers.
It is an object of the present invention is to provide a new and improved method to compensate for variations in the removal of material from material layers on a wafer during CMP.
Another object of the present invention is to provide a new and improved method for achieving precision in the thickness of layers on a wafer using a CMP operation.
Still another object of the present invention is to provide a new and improved method for calculating a compensated removal rate for the removal of material from a layer on a wafer.
Yet another object of the present invention is to provide a method for achieving precise thickness of a material layer on a wafer during CMP.
A still further object of the present invention is to provide a method which can be used to remove material of any needed thickness over a continuum of thicknesses from a layer on a wafer to compensate for over-polishing or under-polishing of the wafer on a CMP apparatus.
Another object of the present invention is to provide a method for CMP removal rate compensation which requires a relatively low quantity of space on a server and is easy to use and maintain.